freedreno/a6xx: viewport-array and gl43

First patch reverts a patch from !19100 (merged) which turned out to be unnecessary

Second adds a nir path alternative to the tgsi path for layered clear for gallium drivers that do not support PIPE_CAP_VS_LAYER_VIEWPORT Dropped this patch, since it turned out to be trivial to enable PIPE_CAP_VS_LAYER_VIEWPORT and the nir path was exposing a problem with virgl (virglrenderer?)

After that some cleanup, code-deletion, and a fix for spec@arb_texture_buffer_object@texture-buffer-size-clamp@* and finally enabling viewport-array. Which also happens to get us all the way to gl43.